1. Field of the Invention
The present invention relates to a method of fabricating an antireflective grating pattern and a method of fabricating an optical device integrated with an antireflective grating pattern, and more particularly, to a method of fabricating an antireflective grating pattern and a method of fabricating an optical device integrated with an antireflective grating pattern in which a wedge-type or parabola-type antireflective subwavelength grating (SWG) pattern having a pointed tip is formed on a semiconductor substrate using a hologram lithography process, a reflow process, and a pattern transfer process to minimize the reflected amount of light caused by a difference in refractive index between the air and a semiconductor material.
2. Discussion of Related Art
In general, reducing the reflected amount of light between two media having different refractive indices is an important problem to be solved in terms of optical devices, such as solar cells, photodetectors, light emitting diodes, or transparent glasses.
FIG. 1 is a conceptual diagram for explaining the reflection and transmission of light when the light is vertically incident on a medium.
Referring to FIG. 1, the reflection of light is determined by a Fresnel equation. Assuming that light is vertically incident on a medium, a reflected amount R is expressed by the following equation 1:
                              R          =                                    (                                                                    n                    1                                    -                                      n                    2                                                                                        n                    1                                    +                                      n                    2                                                              )                        2                          ,                            (        1        )            
wherein n1 and n2 refer to the refractive indices of media. Since a semiconductor material (e.g., silicon (Si) or gallium arsenide (GaAs)) applied to a semiconductor substrate of an optical device has a refractive index n2 of about 3 to 4 and the air has a refractive index n1 of 1, when light is incident from the air on the optical device, about 30% or more of the light may be reflected. Also, when light is emitted from the optical device into the air, similar loss may be generated.
The reflection of the light may be a main cause for a reduction in the efficiency of the optical device. An antireflective coating (ARC) method may be typically used to reduce the reflection of light. The ARC method may include depositing a material, such as a dielectric material or a polymer material, which has a lower refractive index than a semiconductor to reduce the reflection of the light.
According to the ARC method, while minimum reflection characteristics may be obtained in a specific wavelength range by appropriately controlling a refractive index and an optical thickness, it may be difficult to find materials appropriate for various semiconductor materials, to consider electrical and thermal properties, and to reduce the reflection of light over a wide spectrum and there may be a great difference in reflectance according to an incident angle of light.
Meanwhile, research has recently been conducted on a method of reducing the reflection of light using a subwavelength grating (SWG) structure, which is based on the following principle. An incident angle according to the grating order of the grating structure is expressed by Equation 2:
                                          sin            ⁢                                                  ⁢                          θ              m                                =                                                    m                ⁢                                                                  ⁢                λ                                            Λ                ⁢                                                                  ⁢                                  n                  2                                                      +                                                            n                  1                                                  n                  2                                            ⁢              sin              ⁢                                                          ⁢                              θ                i                                                    ,                            (        2        )            
wherein m denotes the grating order, λ denotes a wavelength of incident light, Λ denotes a period of the grating structure, and n1 and n2 denote refractive indices of media.
When the period Λ of the grating structure is much less than the wavelength λ of the incident light, the grating order m can only be 0. In other words, no light is diffracted in a lateral direction. In this case, according to the effective medium theory, the refractive index of the grating structure may be regarded as an effective refractive index in proportion to a fill factor of the grating structure.
For example, when a grating structure having a fill factor of 0.5 is formed using a material having a refractive index of 3, since the refractive index of the air is 1, the effective refractive index becomes 2 according to the equation: (3*0.5+1*0.5)/2. When the grating structure is formed in a cone or pyramid shape rather than a simple rod or ridge shape, a refractive index may be gradually varied from a semiconductor toward the air, and thus the reflection of light may be hardly eliminated.
Meanwhile, conventional methods of fabricating SWG structures include 1) a method of forming a subwavelength pattern using an electron beam (e-beam) lithography process or hologram lithography process and forming a grating structure in a cone shape using a dry etching process or wet etching process (Reference document: Y. Kanamori et. al., Jpn. J. Appl. Phys. 39, L735 (2000), K. Kintaka et. al., Opt. Lett. 26, 1642 (2001), D. L. Brundrett et. al., Appl. Opt. 37, 2534 (1998)), 2) a method of fabricating a pyramidal grating structure using colloidal crystals (Reference document: C. Sun et. al., Appl. Phys. Lett. 91, 231105 (2007)), 3) a method of fabricating a grating structure using nano-imprint and lift-off processes (Reference document: Z. Yu et. al., J. Vac. Sci. Technol. B21, 2874 (2003)), and 4) a method of fabricating an SWG structure using a self-masked etching process (Reference document: Y. Huang et. al., Nat. Nanotechnol. 2, 770 (2007)). However, the above-described conventional methods involve performing complicated processes or employing complicated gas mixtures and preclude forming aligned structures.
In addition, the following two points should be considered during the fabrication of an antireflective SWG structure. First, the packing density of the antireflective SWG structure should be increased. Second, the height and period of the grating structure should be controllable.
When the packing density of the antireflective SWG structure is low, since a difference in refractive index between a semiconductor and the grating structure is great, the reflection of light may be increased. Also, it should be capable of controlling the height and period of the grating structure to control the reflectance of light in a desired wavelength.